In web coating, printing and drying operations, it is often desirable that the web have contactless support, in order to avoid damage to the web itself or to the coating (such as ink) previously applied to one or more surfaces of the web. One conventional arrangement for contactlessly supporting a web during drying includes horizontal upper and lower sets of air bars between which the web travels. Hot air issuing from the air bars both dries and supports the web as it travels through the dryer.
Important characteristics of any flotation system are the amount of cushioning provided by the flotation device, and the stability of the web as it passes over the device. Adequate support removes web wrinkles that typically are caused by higher tensions in lightweight webs. Airflow instabilities near the web can induce web flutter and subsequent web contact with mechanical parts of the dryer, resulting in coating disturbance or web damage. Web flutter can be manifested in a multitude of forms, ranging from a violent flapping of the web to a high frequency drumming.
Single slot air bars are known in the art as air foils. They differ from double and triple slot air bars in that they have both a positive and negative pressure on the face of the air bar, whereas the double and triple slotted bars have only positive pressure. As a result, double and triple slotted air bars can be operated over a wider range of pressures and clearances; typical flotation clearances of air foils being about 2.3 mm compared to 6.3 mm for double and triple air bars. Air foils also have a dramatic decrease in both heat transfer and flotation stability as clearance is increased, whereas the heat transfers for double and triple air bars are relatively stable up to a clearance of 25 mm (single size bar). A typical application for single slot air bars is where flotation must be accomplished with air on only one side of the web.
Conventional air foils discharge air at about 45° to the web, which pushes the web up and relies on the flatness of the web to trap the air and force it to follow the air foil face. This creates a negative pressure to pull the web back down and hold it in place over the air foil. When floating lightweight webs under medium to high tensions, machine direction corrugations will form in the web. These corrugations allow the discharged air from the 45° slot to escape and not trap the air between the air foil face and the web, thereby reducing or eliminating the velocity created to draw the web down to the air foil face. This can result in poor flotation and can render the air foil ineffective.
It is therefore an object of the present invention to provide an air foil for floating a web that provides excellent web support for a wide range of web weights, and provides excellent web stability.